In the face of developing coronary artery occlusion the collateral circulation is a special and extreme example of physiological adaptation. The collateral circulation, following an occlusion, can expand 20 fold within a few short months. It is the most important natural force for preserving the viability of the heart muscle and its function to provide the human body with its metabolic needs. The quantitative study of the magnitude and time course of collateral development following coronary occlusion provides us with the basis against which we can compare the efficacy of drugs, surgical interventions and exercise. Knowledge of the mechanism(s) involved in stimulating the growth and proliferation of collateral vessels would eventually lead to ways to control these factors and perhaps the discovery of drugs affecting this control. Histological and electrocardiographic studies combined with these basic hemodynamic studies will aid in obtaining a more comprehensive view of the problem. The use of the computer as a tool for understanding and integrating some of the complex interrelationships of the various parameters governing the coronary circulation helps us in gaining further insight into the problem.